1. Field of the Invention
The present invention relates to a device and a driving method thereof. Specifically, the present invention relates to a semiconductor device and a driving method thereof. More specifically, the present invention relates to a display device and a driving method thereof, particularly a method of improving the quality of moving images by hold driving.
2. Description of the Related Art
In recent years, there has been growing interests in thin display devices. As substitutes for CRT displays, liquid crystal displays, plasma displays, projection displays, and the like have been developed and becoming popular. Furthermore, field emission displays, inorganic electroluminescence displays, organic electroluminescence displays, electronic paper, and the like have been developed as next-generation display devices.
In a display portion which is provided in the above-described display device, pixels each of which is a minimum unit for constituting an image are juxtaposed. Each of the pixels emits light with certain luminance in accordance with the image data, whereby an image is formed in the display portion.
When a moving image is to be displayed with the use of such a display device, different images are displayed quickly, several tens of times per second. The period in which an image is displayed is referred to as one frame period.
Here, driving methods of display devices can be classified according to temporal distribution of the luminance of a pixel in one frame period. As for the hold driving which is typified by an active matrix display device, the luminance of a pixel is constant in one frame period. On the other hand, as for impulsive driving which is typified by CRT, after intense light is emitted once in one frame period, the luminance of a pixel immediately attenuates and light is not emitted any more. In impulsive driving, a non-lighting state dominates the most part of one frame period.
The recent research has made it clear that the hold driving has an essential problem in that contour becomes vague or movement looks unnatural when a moving image is displayed. An impulse-driven display device does not have this problem. In order to solve such a problem unique to the hold driving, a method for realizing pseudo impulsive driving, in which a black image that displays nothing is displayed for a certain period in one frame period, is disclosed (for example, Reference 1: Japanese Published Patent Application No. H9-325715 and Reference 2: Japanese Published Patent Application No. 2000-200063). In addition, as another method for realizing pseudo impulsive driving, Reference 3: SID'05 DIGEST, 60.2, pp 1734, (2005) is disclosed. This is a method in which one frame period is divided into two subframe periods, light is emitted only in a subframe that is located in the latter half of one frame in a low gray scale region, and light is emitted in a subframe that is located in the first half of one frame while light is emitted in a subframe located in the latter half of one frame in a high gray scale region, thereby realizing a pseudo impulsive driving.